1. Field of the Invention
The present invention relates to a catalytic process for preparing nitriles from amides.
2. Description of the Prior Art
It is known for instance from U.S. Pat. Nos. 3,256,311, 3,275,581 and 3,514,478, from German Pat. Nos. 1,086,710 and 1,908,967 and from French Pat. Nos. 1,536,750 and 1,250,165 that nitriles may be formed from N-alkylformamides, N-arylformamides, and .alpha.,.omega.-diformamides. U.S. Pat. No. 3,275,581 also discloses the formation of nitriles from primary amines in the presence of formic acid or formates.
The catalysts used in these known processes are silica gels of various porosities. They may be activated with up to 10% of oxides of metals selected from groups III-A, IV-B, V-B and VI-B of the periodic classification of elements.
These catalysts have a relative short active life, even if they can sometimes be regenerated.
It has been tried to prevent this catalyst die-off and to increase their active life by various techniques: by working under reduced pressure (according for instance to German Pat. No. 1,908,967), by using an inert gas such as NH.sub.3, CO or N.sub.2 (according to for instance U.S. Pat. No. 3,275,581 and German Pat. No. 1,086,710) or by dissolving the formamides in solvents such as nitriles of fatty acids or benzene-carboxylic acids or in other alkylformamides than the formamide which is reacted (according to for instance French Pat. No. 1,536,750).
None of these techniques has however given very interesting results and both the catalyst die-off and the catalytic activity and selectivity have caused drawbacks to these processes.
On the other hand, there are known catalysts for the oxidation and ammoxidation or ammonoxidation of propylene and other aliphatic olefins and for the ammoxidation or ammonoxidation of aromatic compounds. Such catalysts are for instance disclosed in Richard Krabetz article in Chemie- Ing.- Techn. 46 (1974) 24, pp. 1029-1041.
Ammoxidation or ammonoxidation catalysts are in fact those catalysts which are known to form a nitrile radical when in contact with a molecule having a methyl radical linked to an aromatic ring or to a carbon atom involved in a carbon-carbon double bond, and with oxygen and ammonia.
The ammoxidation or ammonoxidation reaction itself is chemically quite different from the reaction in which nitriles are prepared from amides. Thus, the ammonoxidation reaction merely changes a CH.sub.3 group into a C.tbd.N group, whereas the conversion of an amide into a nitrile involves a transposition of the hydrocarbon chain of the molecule, and generally results in the addition of one carbon atom to said hydrocarbon chain.